Durable fuel cell membrane electrode assembly with combined additives

ABSTRACT

A fuel cell membrane electrode assembly is provided comprising a polymer electrolyte membrane comprising a first polymer electrolyte and at least one manganese compound; and one or more electrode layers comprising a catalyst and at least one cerium compound. The membrane electrode assembly demonstrates an unexpected combination of durability and performance.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 61/017,027, filed Dec. 27, 2007, U.S. patent applicationSer. No. 12/342,370, filed Dec. 23, 2008, now U.S. Pat. No. 8,137,828,granted Mar. 20, 2012, and U.S. patent application Ser. No. 13/396,093,filed Feb. 14, 2012, now allowed, the disclosures of each of which areincorporated by reference herein in their entireties.

FIELD OF THE DISCLOSURE

This disclosure relates to fuel cell membrane electrode assemblies(MEA's) with highly improved durability and performance. The MEA's ofthe present disclosure include one or more electrodes which include oneor more cerium oxides and polymer electrolyte membranes (PEM's) whichinclude one or more manganese cations or manganese compounds.

BACKGROUND OF THE DISCLOSURE

U.S. Pat. App. Pub. Nos. 2006/0063054 A1 and 2006/0063055 A1 and U.S.patent application Ser. Nos. 11/261,053, 11/262,268 and 11/962,971,incorporated herein by reference, address the use of cerium or manganeseadditives to improve durability of polymer electrolyte membranes(PEM's).

SUMMARY OF THE INVENTION

Briefly, the present disclosure provides a fuel cell membrane electrodeassembly comprising a polymer electrolyte membrane comprising a firstpolymer electrolyte and at least one manganese cation or manganesecompound; and one or more electrode layers comprising a catalyst and atleast one cerium compound. In some embodiments, the first polymerelectrolyte comprises bound anionic functional groups, at least aportion of said anionic functional groups are in acid form and at leasta portion of said anionic functional groups are neutralized by manganesecations. In some embodiments, the distribution of manganese throughoutthe polymer electrolyte membrane is uniform. In some embodiments themanganese cations are Mn²⁺ cations. The one or more electrode layers mayadditionally comprise a second polymer electrolyte. The first polymerelectrolyte, second polymer electrolyte or both may optionally be highlyfluorinated or perfluorinated. The first polymer electrolyte and thesecond polymer electrolyte may have the same or different composition.The first polymer electrolyte and the second polymer electrolyte mayhave the same or different equivalent weight. The first polymerelectrolyte and the second polymer electrolyte may have the same ordifferent molecular weight. In some embodiments the amount of manganesecations present is between 0.001 and 0.5 charge equivalents based on themolar amount of anionic functional groups present in the first polymerelectrolyte, and in some embodiments between 0.01 and 0.1 chargeequivalents. In some embodiments the first polymer electrolyte has anequivalent weight of 1050 or less, in some embodiments 1000 or less, insome embodiments 950 or less, in some embodiments 900 or less, in someembodiments 850 or less, and in some embodiments 800 or less. In someembodiments the first polymer electrolyte comprises pendent groupsaccording to the formula: —O—CF₂—CF₂—CF₂—CF₂—SO₃H. In some embodimentsthe MEA additionally comprises a porous support. In some embodiments theone or more electrode layers comprise cerium compounds in an amount ofbetween 0.01 and 5 percent by weight relative to the weight of catalyst,more typically between 0.1 and 1.0 percent by weight. In someembodiments the cerium compound is a cerium oxide compound. In someembodiments the cerium oxide compound is CeO₂. In some embodiments thecerium oxide compound is Ce₂O₃.

What has not been described in the art, and is provided by the presentinvention, is a fuel cell membrane electrode assembly comprising aspecific combination of additives, including specificity of location andchemical form, which provides an unprecedented and unexpectedcombination of performance and durability.

In this application:

“uniform” distribution of an additive in a polymer membrane means thatthe amount of additive present does not vary more than +/−90%, moretypically not more than +/−50% and more typically not more than +/−20%;

“equivalent weight” (EW) of a polymer means the weight of polymer whichwill neutralize one equivalent of base;

“polyvalent cation” means a cation having a charge of 2+ or greater;

“highly fluorinated” means containing fluorine in an amount of 40 wt %or more, typically 50 wt % or more and more typically 60 wt % or more;and

“acid form” means, with regard to an anionic functional group, that itis neutralized by a proton.

It is an advantage of the present invention to provide a fuel cellmembrane electrode assembly demonstrating both high performance and highdurability.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph of fuel cell performance (voltage) vs. fluoriderelease (a measure of durability) for Comparative MEA's A, B, C and Dand a fuel cell membrane assembly according to the present disclosure,MEA E, as described further in the Examples.

DETAILED DESCRIPTION

The present disclosure provides membrane electrode assemblies (MEA's)with exceptionally high durability, particularly under hot and dryconditions, that is conditions above 80° C. and/or below 75% RH. TheMEA's of the present disclosure include one or more electrodes whichinclude one or more cerium oxides, typically present as dispersedparticles, and in some embodiments, PEM's which include one or moremanganese salts.

A membrane electrode assembly (MEA) or polymer electrolyte membrane(PEM) according to the present disclosure may be useful in anelectrochemical cell such as a fuel cell. An MEA is the central elementof a proton exchange membrane fuel cell, such as a hydrogen fuel cell.Fuel cells are electrochemical cells which produce usable electricity bythe catalyzed combination of a fuel such as hydrogen and an oxidant suchas oxygen. Typical MEA's comprise a polymer electrolyte membrane (PEM)(also known as an ion conductive membrane (ICM)), which functions as asolid electrolyte. One face of the PEM is in contact with an anodeelectrode layer and the opposite face is in contact with a cathodeelectrode layer. In typical use, protons are formed at the anode viahydrogen oxidation and transported across the PEM to the cathode toreact with oxygen, causing electrical current to flow in an externalcircuit connecting the electrodes. Each electrode layer includeselectrochemical catalysts, typically including platinum metal. The PEMforms a durable, non-porous, electrically non-conductive mechanicalbarrier between the reactant gases, yet it also passes H⁺ ions readily.Gas diffusion layers (GDL's) facilitate gas transport to and from theanode and cathode electrode materials and conduct electrical current.The GDL is both porous and electrically conductive, and is typicallycomposed of carbon fibers. The GDL may also be called a fluid transportlayer (FTL) or a diffuser/current collector (DCC). In some embodiments,the anode and cathode electrode layers are applied to GDL's and theresulting catalyst-coated GDL's sandwiched with a PEM to form afive-layer MEA. The five layers of a five-layer MEA are, in order: anodeGDL, anode electrode layer, PEM, cathode electrode layer, and cathodeGDL. In other embodiments, the anode and cathode electrode layers areapplied to either side of the PEM, and the resulting catalyst-coatedmembrane (CCM) is sandwiched between two GDL's to form a five-layer MEA.The terms “electrode layer” and “catalyst layer” are usedinterchangeably as used herein.

The PEM according to the present disclosure may comprise any suitablepolymer electrolyte. The polymer electrolytes useful in the presentdisclosure typically bear anionic functional groups bound to a commonbackbone, which are typically sulfonate groups (forming sulfonic acidgroups when neutralized by protons) but may also include carboxylategroups (forming carboxylic acid groups when neutralized by protons),deprotonated imide groups, deprotonated sulfonamide groups, anddeprotonated amide groups, or other functional groups that form acidswith protonated. The polymer electrolytes useful in the presentdisclosure typically are highly fluorinated and most typicallyperfluorinated. The polymer electrolytes useful in the presentdisclosure are typically copolymers of tetrafluoroethylene and one ormore fluorinated, acid-functional comonomers. Typical polymerelectrolytes include Nafion® (DuPont Chemicals, Wilmington Del.) andFlemion™ (Asahi Glass Co. Ltd., Tokyo, Japan). The polymer electrolytemay be a copolymer of tetrafluoroethylene (TFE) andFSO₂—CF₂CF₂CF₂CF₂—O—CF═CF₂, described in U.S. patent application Ser.Nos. 10/322,254, 10/322,226 and 10/325,278, which are incorporatedherein by reference. The polymer typically has an equivalent weight (EW)of 1200 or less and more typically 1100 or less. In some embodiments,polymers of unusually low EW can be used, typically 1000 or less, moretypically 900 or less, and more typically 800 or less, often withimproved performance in comparison to the use of higher EW polymer.

The polymer can be formed into a membrane by any suitable method. Thepolymer is typically cast from a suspension. Any suitable casting methodmay be used, including bar coating, spray coating, slit coating, brushcoating, and the like. Alternately, the membrane may be formed from neatpolymer in a melt process such as extrusion. After forming, the membranemay be annealed, typically at a temperature of 120 C or higher, moretypically 130 C or higher, most typically 150 C or higher. In someembodiments of the method according to the present disclosure, additivesare added to the membrane only after annealing and not before, andtherefore annealing conditions are not impacted by their presence, whichmay, e.g., raise membrane Tg, thus necessitating higher annealingtemperatures. The PEM typically has a thickness of less than 50 microns,more typically less than 40 microns, more typically less than 30microns, and most typically about 25 microns.

In some embodiments of the present disclosure, one or more manganesecompounds are added to the polymer electrolyte of the PEM prior to,during or after membrane formation. In some embodiments of the presentdisclosure, one or more manganese salts are added to the polymerelectrolyte of the PEM prior to, during or after membrane formation. Insome embodiments, one or more manganese salts are added to a dispersionof the polymer electrolyte of the PEM prior to casting to form amembrane. In some embodiments, a membrane coated with or immersed in asolution of one or more manganese salts.

Manganese salts may comprise any suitable anion, including chloride,bromide, nitrate, carbonate and the like. Once cation exchange occursbetween the transition metal salt and the acid form polymer, it may bedesirable for the acid formed by combination of the liberated proton andthe original salt anion to be removed. Thus, it may be preferred to useanions that generate volatile or soluble acids, for example chloride ornitrate. Manganese cations may be in any suitable oxidation state, butare most typically Mn²⁺. Without wishing to be bound by theory, it isbelieved that the manganese cations persist in the polymer electrolytebecause they are exchanged with H⁺ ions from the anion groups of thepolymer electrolyte and become associated with those anion groups.Furthermore, it is believed that polyvalent manganese cations may formcrosslinks between anion groups of the polymer electrolyte, furtheradding to the stability of the polymer.

The amount of salt added is typically between 0.001 and 0.5 chargeequivalents (of Manganese cations) based on the molar amount of anionicfunctional groups present in the polymer electrolyte, more typicallybetween 0.005 and 0.2, more typically between 0.01 and 0.1, and moretypically between 0.02 and 0.05.

In some embodiments of the present disclosure, a membrane electrodeassembly comprises a polymer electrolyte membrane with bound anionicfunctional groups, wherein at least a portion of said anionic functionalgroups are in acid form and at least a portion of said anionicfunctional groups are neutralized by manganese cations. Examples of suchmanganese cations include Mn²⁺, Mn³⁺, Mn⁴⁺, Mn⁵⁺, and Mn⁶⁺. In somepreferred embodiments, the manganese cations are Mn²⁺ cations. Theportion of the anionic functional groups that are neutralized bymanganese cations is typically between 0.001 and 0.5 charge equivalentsbased on the molar amount of anionic functional groups present in thepolymer electrolyte, more typically between 0.005 and 0.2, moretypically between 0.01 and 0.1, and more typically between 0.02 and0.05.

A PEM according to the present disclosure may additionally comprise aporous support, such as a layer of expanded PTFE or the like, where thepores of the porous support contain the polymer electrolyte. A PEMaccording to the present disclosure may comprise no porous support. APEM according to the present disclosure may comprise a crosslinkedpolymer.

To make an MEA or CCM, catalyst may be applied to the PEM by anysuitable means, including both hand and machine methods, including handbrushing, notch bar coating, fluid bearing die coating, wire-wound rodcoating, fluid bearing coating, slot-fed knife coating, three-rollcoating, or decal transfer. Coating may be achieved in one applicationor in multiple applications.

Any suitable catalyst may be used in the practice of the presentdisclosure. Typically, carbon-supported catalyst particles are used.Typical carbon-supported catalyst particles are 50-90% carbon and 10-50%catalyst metal by weight, the catalyst metal typically comprising Pt forthe cathode and anode. In some embodiments, the catalyst metal comprisesPt and Ru in a weight ratio of 2:1 for the anode. Typically, thecatalyst is applied to the PEM or to the FTL in the form of a catalystink. Alternately, the catalyst ink may be applied to a transfersubstrate, dried, and thereafter applied to the PEM or to the FTL as adecal. The catalyst ink typically comprises polymer electrolytematerial, which may or may not be the same polymer electrolyte materialwhich comprises the PEM. The catalyst ink typically comprises adispersion of catalyst particles in a dispersion of the polymerelectrolyte. The ink typically contains 5-30% solids (i.e. polymer andcatalyst) and more typically 10-20% solids. The electrolyte dispersionis typically an aqueous dispersion, which may additionally containalcohols and polyalcohols such a glycerin and ethylene glycol. Thewater, alcohol, and polyalcohol content may be adjusted to alterrheological properties of the ink. The ink typically contains 0-50%alcohol and 0-20% polyalcohol. In addition, the ink may contain 0-2% ofa suitable dispersant. The ink is typically made by stirring with heatfollowed by dilution to a coatable consistency.

In some embodiments of the present disclosure, one or more ceriumcompounds are added to one or both of the anode or cathode catalystlayers of the MEA, prior to, during or after formation of the electrodelayer. In some embodiments of the present disclosure, one or more ceriumoxide compounds are added to one or both of the anode or cathodecatalyst layers of the MEA, prior to, during or after formation of theelectrode layer.

Cerium oxide compounds may be crystalline or amorphous. The cerium oxidecompound may contain cerium in the (IV) oxidation state, the (III)oxidation state, or both. In one embodiment, the cerium oxide compoundis substantially free of metallic cerium. Alternately, the cerium oxidecompound may contain both cerium oxide and metallic cerium. Essentiallypure cerium (IV) oxide is represented by the chemical formula CeO₂.Essentially pure cerium (III) oxide is represented by the chemicalformula Ce₂O₃. Alternately, the cerium oxide compound may be supportedas a thin oxidation reaction product layer on a metallic ceriumparticle. In one embodiment, the cerium oxide compound may containsubstantially no other metal elements. Alternately, the cerium oxidecompound may contain one or more other metal elements and thus may beconsidered a mixed metal oxide compound comprising cerium oxide.Examples of mixed metal oxide compounds comprising cerium oxide includesolid solutions such as zirconia-ceria and multicomponent oxidecompounds such as barium cerate. “Cerium oxides” and “cerium oxidecompounds” are used interchangeably herein and include to theaforementioned substances. The amount of cerium oxide present in theelectrode layer is not limited, except to the extent that largeadditions can interfere with the electrochemical performance of theelectrode. The amount of cerium oxide compound added is typicallybetween 0.01 and 5 weight percent based on the total weight of thesupported catalyst in the electrode layer, more typically between 0.1and 2 wt %, and more typically between 0.2 and 1.3 wt %. The ceriumoxide compound is typically present in an amount of less than 1% byvolume relative to the total volume of the polymer electrolyte in theelectrode layer, more typically less than 0.8% by volume, and moretypically less than 0.5% by volume. Cerium oxide may be in the form ofparticles of any suitable size, typically between 1 and 5000 nm. In someembodiments, particle sizes of 200-5000 nm are preferred. In someembodiments, particle sizes of 500-1000 nm are preferred. Other usefulcerium compounds include cerium salts and cerium metalorganic compounds,including cerium carboxylates, as well as mixed-metal analogues thatinclude cerium. Examples of other useful cerium compounds include ceriumacetate, cerium nitrate, cerium sulfate, and cerium phosphate.

In some embodiments, the membrane electrode assembly according to thepresent disclosure contains at least one manganese cation or compound inthe polymer electrolyte membrane and at least one cerium compound in oneor more electrode layers. In some embodiments, the membrane electrodeassembly according to the present disclosure contains manganese cationsin the polymer electrolyte membrane and at least one cerium compound inone or more electrode layers. In some embodiments, the membraneelectrode assembly according to the present disclosure contains apolymer electrolyte membrane comprising a polymer electrolyte with boundanionic functional groups wherein a portion of said groups are in acidform and a portion are neutralized by manganese cations, the membraneelectrode assembly also containing at least one cerium compound in oneor more electrode layers. In some embodiments, the membrane electrodeassembly according to the present disclosure contains at least onemanganese compound in the polymer electrolyte membrane and cerium oxidecompounds in one or more electrode layers. In some embodiments, themembrane electrode assembly according to the present disclosure containsmanganese cations in the polymer electrolyte membrane and at least onecerium oxide compound in one or more electrode layers. In someembodiments, the membrane electrode assembly according to the presentdisclosure contains a polymer electrolyte membrane comprising a polymerelectrolyte with bound anionic functional groups wherein a portion ofsaid groups are in acid form and a portion are neutralized by manganesecations, the membrane electrode assembly also containing at least onecerium oxide compound in one or more electrode layers. In someembodiments, the membrane electrode assembly according to the presentdisclosure contains Mn²⁺ cations in the polymer electrolyte membrane andCeO₂ in one or more electrode layers. In some embodiments, the membraneelectrode assembly according to the present disclosure contains apolymer electrolyte membrane comprising a polymer electrolyte with boundanionic functional groups wherein a portion of said groups are in acidform and a portion are neutralized by manganese cations, the membraneelectrode assembly also containing CeO₂ in one or more electrode layers.In some embodiments, the membrane electrode assembly according to thepresent disclosure contains Mn²⁺ cations in the polymer electrolytemembrane and Ce₂O₃ in one or more electrode layers. In some embodiments,the membrane electrode assembly according to the present disclosurecontains a polymer electrolyte membrane comprising a polymer electrolytewith bound anionic functional groups wherein a portion of said groupsare in acid form and a portion are neutralized by manganese cations, themembrane electrode assembly also containing Ce₂O₃ in one or moreelectrode layers. In some embodiments, the membrane electrode assemblycontains at least 50% less cerium in the PEM than in the electrodes. Insome embodiments, the membrane electrode assembly contains at least 90%less cerium in the PEM than in the electrodes. In some embodiments, themembrane electrode assembly contains at least 95% less cerium in the PEMthan in the electrodes. In some embodiments, the membrane electrodeassembly contains at least 99% less cerium in the PEM than in theelectrodes. In some embodiments, the membrane electrode assemblycontains no cerium in the PEM. In some embodiments, the membraneelectrode assembly contains at least 50% less manganese in theelectrodes than in the PEM. In some embodiments, the membrane electrodeassembly contains at least 90% less manganese in the electrodes than inthe PEM. In some embodiments, the membrane electrode assembly containsat least 95% less manganese in the electrodes than in the PEM. In someembodiments, the membrane electrode assembly contains at least 99% lessmanganese in the electrodes than in the PEM. In some embodiments, themembrane electrode assembly contains no manganese in the electrodes.

In some embodiments, the membrane electrode assembly according to thepresent disclosure contains a polymer electrolyte membrane comprising apolymer electrolyte with bound anionic functional groups wherein aportion of said groups are in acid form and wherein between 0.001 an 0.5charge equivalents of said groups are neutralized by manganese cations,the membrane electrode assembly also containing 0.01 and 5 weightpercent CeO₂ in one or more electrode layers based on the total weightof the supported catalyst in the electrode layer or layers. In someembodiments, the membrane electrode assembly according to the presentdisclosure contains a polymer electrolyte membrane comprising a polymerelectrolyte with bound anionic functional groups wherein a portion ofsaid groups are in acid form and wherein between 0.01 an 0.1 chargeequivalents of said groups are neutralized by manganese cations, themembrane electrode assembly also containing 0.1 and 2 weight percentCeO₂ in one or more electrode layers based on the total weight of thesupported catalyst in the electrode layer or layers.

To make an MEA, GDL's may be applied to either side of a CCM by anysuitable means. Any suitable GDL may be used in the practice of thepresent disclosure. Typically the GDL is comprised of sheet materialcomprising carbon fibers. Typically the GDL is a carbon fiberconstruction selected from woven and non-woven carbon fiberconstructions. Carbon fiber constructions which may be useful in thepractice of the present disclosure may include: Toray™ Carbon Paper,SpectraCarb™ Carbon Paper, AFN™ non-woven carbon cloth, Zoltek™ CarbonCloth, and the like. The GDL may be coated or impregnated with variousmaterials, including carbon particle coatings, hydrophilizingtreatments, and hydrophobizing treatments such as coating withpolytetrafluoroethylene (PTFE).

In use, the MEA according to the present disclosure is typicallysandwiched between two rigid plates, known as distribution plates, alsoknown as bipolar plates (BPP's) or monopolar plates. Like the GDL, thedistribution plate must be electrically conductive. The distributionplate is typically made of a carbon composite, metal, or plated metalmaterial. The distribution plate distributes reactant or product fluidsto and from the MEA electrode surfaces, typically through one or morefluid-conducting channels engraved, milled, molded or stamped in thesurface(s) facing the MEA(s). These channels are sometimes designated aflow field. The distribution plate may distribute fluids to and from twoconsecutive MEA's in a stack, with one face directing fuel to the anodeof the first MEA while the other face directs oxidant to the cathode ofthe next MEA (and removes product water), hence the term “bipolarplate.” Alternately, the distribution plate may have channels on oneside only, to distribute fluids to or from an MEA on only that side,which may be termed a “monopolar plate.” The term bipolar plate, as usedin the art, typically encompasses monopolar plates as well. A typicalfuel cell stack comprises a number of MEA's stacked alternately withbipolar plates.

This disclosure is useful in the manufacture and operation of fuelcells.

Objects and advantages of this disclosure are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this disclosure.

EXAMPLES

Unless otherwise noted, all reagents were obtained or are available fromAldrich Chemical Co., Milwaukee, Wis., or may be synthesized by knownmethods.

Four types of Comparative MEA (A, B, C and D) and one MEA of the presentdisclosure (E) were made. FIG. 1 is a graph of fuel cell performance(voltage) vs. fluoride release (a measure of durability) for ComparativeMEA's A, B, C and D and a fuel cell membrane assembly according to thepresent disclosure, MEA E. Results reported for Comparative Example Crepresent the average result for 2 MEA's.

TABLE I Fluoride Perfor- Membrane Electrode release mance MEA PEMAdditive Additive (μg/daycm²) (V) A 3M Mn(NO₃)₂ none 4.35 0.667(Comparative) Ionomer 3.5% B 3M Mn(NO₃)₂ none 0.11 0 (Comparative)Ionomer Saturated C Nafion none 1% CeO₂ 9.07 0.67 (Comparative) D Nafionnone 10% 0.21 0.359 (Comparative) CeO₂ E 3M Mn(NO₃)₂ 1% CeO₂ 0.66 0.634Ionomer 3.5%Membranes

The polymer electrolyte membranes (PEM's) used in the Example andComparative Examples were, as noted in Table I, one of: a) 25 micronthick membranes cast from Nafion® SE 2009 solution (DuPont Chemicals,Wilmington Del.), reported equivalent weight (EW) 920, or b) 20 micronthick membranes cast from 3M Ionomer, which was a copolymer oftetrafluoroethylene (TFE) and FSO₂—CF₂CF₂CF₂CF₂—O—CF═CF₂ (Comonomer A).Comonomer A was made according to the procedures disclosed in U.S.patent application Ser. Nos. 10/322,254 and 10/322,226, incorporatedherein by reference. Polymerization was performed by aqueous emulsionpolymerization as described in U.S. patent application Ser. No.10/325,278. The equivalent weight (EW) was 800. The ionomer was providedin a casting solution containing 16.7% solids in 70:30 n-propanol/water.

Membranes were cast from solution as disclosed in U.S. Pat. App. Pub.Nos. 2006/0063054 A1 and 2006/0063055 A1 and U.S. patent applicationSer. Nos. 11/261,053, 11/262,268, the disclosures of which areincorporated herein by reference. Membranes were cast at a wet thicknessof about 400 to 500 microns, onto a substrate of either PET(polyethylene terphthalate) or Kapton (polyimide). These castings weredried at 80-100° C., and then annealed at 160-200° C. for about 3 to 5minutes. After cooling, the membranes were peeled form the liner andused without further purification. For MEA E and Comparative MEA A,Mn(NO₃)₂ was added to the casting solution in an amount equal to 0.035charge equivalents based on the molar amount of anionic functionalgroups present in the polymer electrolyte. For Comparative MEA B, themembrane was soaked in Mn(NO₃)₂ solution after casting to produce amembrane completely saturated in Mn(NO₃)₂.

Membrane Electrode Assemblies (MEA's)

MEA's having 50 cm² of active area were made from membranes as disclosedin U.S. Pat. App. Pub. Nos. 2006/0063054 A1 and 2006/0063055 A1 and U.S.patent application Ser. Nos. 11/261,053, 11/262,268, the disclosures ofwhich are incorporated herein by reference. Catalyst dispersions wereprepared according to the method described in WO 2002/061,871,incorporated herein by reference. For MEA E and Comparative MEA C, CeO₂was added to the catalyst dispersion in an amount that was 1% by weightof the amount of supported catalyst. For Comparative MEA D, CeO₂ wasadded to the catalyst dispersion in an amount that was 10% by weight ofthe amount of catalyst. To prepare catalyst-coated membranes, anode andcathode layers were applied to membranes according to the decal transfermethod described in WO 2002/061,871. The catalyst loading was 0.6 mg/cm²on both anode and cathode for all MEA's. Where CeO₂ was added to theelectrodes, it was added to both anode and cathode. PTFE-treated carbonpaper gas diffusion layers and polytetrafluoroethylene/glass compositegaskets were applied to the CCM by pressing in a Carver Press (FredCarver Co., Wabash, Ind.) with 13.4 kN of force at 132° C. for 10minutes to complete the MEA's.

Performance Test

All MEA's were tested in a test station with independent controls of gasflow, pressure, relative humidity, and current or voltage (Fuel CellTechnologies, Albuquerque, N. Mex.). The test fixture included graphitecurrent collector plates with quad-serpentine flow fields. Samples wereconditioned overnight and subsequently tested in an air utilization testwherein performance is measured as cathode stoichiometry is graduallyreduced. Performance at the lowest air stoichiometry, 1.3 in this case,is taken as a measure for how well the catalyst and membrane performtogether. The performance metric is the measured voltage at 0.6 A/cm² atthe following conditions: H₂/Air flow rates 600/994 sccm (1.3stoichiometry on cathode) 0/0 PSIG, Cell Temperature=70° C., Inlet DewPoints 70/70° C. (100/100% RH), Current held constant at 0.6 A/cm². Themeasured voltages for MEA E and Comparative MEA's A, B C, and D arereported in Table 1 and plotted in FIG. 1.

Fluoride Release Test

Fluoride release is measured as an indication of durability, with lowfluoride release correlating with high durability.

After the performance metric was measured, MEA's were run under thefollowing conditions: H₂/Air flow rates 657/1657 sccm, 7.25/7.25 PSIG,Cell Temperature=95° C., Inlet Dew Points 77.1/77.1° C. (60/50% RH),Continually scan OCV until OCV drops below 800 mV. Effluent water wascollected daily until samples failed and fluoride content of theeffluent water was measured with an ion chromatograph. The average dailyfluoride release over the lifetime of the sample was calculated for MEAE and Comparative MEA's A, B C, and D and are reported in Table 1 andplotted in FIG. 1.

Evaluation

Performance and fluoride release data for MEA E and Comparative MEA's A,B C, and D are plotted in FIG. 1. Line 1 connects data points forComparative MEA's C and D, which were made with CeO₂ electrode additive.Line 2 connects data points for Comparative MEA's A and B, which weremade with manganese PEM additive. It can be readily seen in FIG. 1 thatexcess addition of either additive improves fluoride release results atthe expense of performance. However, MEA E simultaneously demonstratesboth low fluoride release and high performance. The results for MEA Eare beyond any that would be expected on the basis of the resultsobtained with MEA's A, B C, and D.

Various modifications and alterations of this disclosure will becomeapparent to those skilled in the art without departing from the scopeand principles of this disclosure, and it should be understood that thisdisclosure is not to be unduly limited to the illustrative embodimentsset forth hereinabove.

We claim:
 1. An electrochemical cell comprising: a) a polymerelectrolyte membrane comprising a first polymer electrolyte and at leastone manganese cation, wherein the distribution of manganese cationsacross the thickness of the polymer electrolyte membrane is uniform, andwherein the distribution of manganese cations throughout the polymerelectrolyte membrane is uniform; and b) at least one electrode layerscomprising a catalyst and at least one compound containing cerium andoxygen, wherein a total amount of compounds containing cerium and oxygenis between 0.01 and 5 percent by weight relative to the weight ofcatalyst present.
 2. The electrochemical cell according to claim 1,wherein the first polymer electrolyte comprises bound anionic functionalgroups, wherein at least a portion of said anionic functional groups arein acid form and at least a portion of said anionic functional groupsare neutralized by manganese cations.
 3. The electrochemical cellaccording to claim 2, wherein said manganese cations are Mn²⁺ cations.4. The electrochemical cell according to claim 1, wherein the firstpolymer electrolyte is highly fluorinated.
 5. The electrochemical cellaccording to claim 1, wherein the first polymer electrolyte isperfluorinated.
 6. The electrochemical cell according to claim 1,wherein the one or more electrode layers additionally comprise a secondpolymer electrolyte.
 7. The electrochemical cell according to claim 6,wherein the second polymer electrolyte is highly fluorinated.
 8. Theelectrochemical cell according to claim 6, wherein the second polymerelectrolyte is perfluorinated.
 9. The electrochemical cell according toclaim 2, wherein the amount of manganese cations present is between0.001 and 0.5 charge equivalents based on the molar amount of anionicfunctional groups present in the first polymer electrolyte.
 10. Theelectrochemical cell according to claim 2, wherein the amount ofmanganese cations present is between 0.01 and 0.1 charge equivalentsbased on the molar amount of anionic functional groups present in thefirst polymer electrolyte.
 11. The electrochemical cell according toclaim 1, wherein said first polymer electrolyte has an equivalent weightof 900 or less.
 12. The electrochemical cell according to claim 1,wherein said first polymer electrolyte comprises pendent groupsaccording to the formula:—O—CF₂—CF₂—CF₂—CF₂—SO₃H.
 13. The electrochemical cell according to claim1, additionally comprising a porous support.
 14. The electrochemicalcell according to claim 1, wherein said cerium oxide compounds are CeO₂.15. The electrochemical cell according to claim 1, wherein said ceriumoxide compounds are Ce₂O₃.
 16. The electrochemical cell according toclaim 1 which contains at least 50% less manganese in the electrodelayer(s) than in the polymer electrolyte membrane and at least 50% lesscerium in the polymer electrolyte membrane than in the electrodelayer(s).
 17. The electrochemical cell according to claim 1, wherein atotal amount of compounds containing cerium and oxygen is between 0.1and 2 percent by weight relative to the weight of catalyst present. 18.The electrochemical cell according to claim 1, wherein a total amount ofcompounds containing cerium and oxygen is between 0.2 and 1.3 percent byweight relative to the weight of catalyst present.